An organic light-emitting diode (OLED) device, also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In single-color OLED devices or displays, also called monochrome OLEDs, these organic layers are not patterned but are formed as continuous layers.
In multicolor OLED devices or displays or in full-color OLED displays, an organic hole-injecting and hole-transporting layer is formed as a continuous layer over and between the first electrodes. A pattern of one or more laterally adjacent organic light-emitting layers are then formed over the continuous hole-injecting and hole-transporting layer. This pattern, and the organic materials used to form the pattern, is selected to provide multicolor or full-color light-emission from a completed and operative OLED display in response to electrical potential signals applied between the first and second electrodes.
An unpatterned organic electron-injecting and electron-transporting layer is formed over the patterned light-emitting layers, and one or more second electrodes are provided over this latter organic layer.
Providing a patterned organic light-emitting layer capable of emitting light of two different colors (multicolor) or of three different colors, for example, the primary colors of red (R), green (G), and blue (B), is also referred to as color pixelation since the pattern is aligned with pixels of an OLED display. The RGB pattern provides a full-color OLED display.
Various processes have been proposed to achieve color pixelation in OLED imaging panels. For example, Tang et al. in commonly assigned U.S. Pat. No. 5,294,869 discloses a process for the fabrication of a multicolor OLED imaging panel using a shadow masking method in which sets of pillars or walls made of electrically insulative materials form an integral part of the device structure. A multicolor organic electroluminescent (“EL”) medium is vapor deposited and patterned by controlling an angular position of a substrate with respect to a deposition vapor stream. The complexity of this process resides in the requirements that the integral shadow mask have multilevel topological features, which may be difficult to produce, and that angular positioning of the substrate with respect to one or more vapor sources must be controlled.
Littman et al. in commonly assigned U.S. Pat. No. 5,688,551 recognized the complexity of the above described Tang et al. process, and discloses a method of forming a multicolor organic EL display panel in which a close-spaced deposition technique is used to form a separately colored organic EL medium on a substrate by patternwise transferring the organic EL medium from a donor sheet to the substrate. The donor sheet includes a radiation-absorbing layer which can be unpatterned or which can be prepatterned in correspondence with a pattern of pixels or subpixels on the substrate. The donor sheet has to be positioned either in direct contact with a surface of the substrate or at a controlled relatively small distance from the substrate surface to minimize the undesirable effect of divergence of the EL medium vapors issuing from the donor sheet upon heating the radiation-absorbing layer.
In general, positioning an element such as, for example, a donor sheet or a mask, in direct contact with a surface of a substrate can invite problems of abrasion, distortion, or partial lifting of a relatively thin and mechanically fragile organic layer which has been formed previously on the substrate surface. For example, an organic hole-injecting and hole-transporting layer may be formed over the substrate prior to deposition of a first-color pattern. In depositing a second-color pattern, direct contact of a donor sheet or a mask with the first-color pattern may cause abrasion, distortion, or partial lifting of the first-color pattern.
Positioning a donor sheet or a mask at a controlled distance from the substrate surface may require incorporation of spacer elements on the substrate, or on the donor sheet or the mask, or on the substrate and on the donor sheet. Alternatively, special fixtures may have to be devised to provide for a controlled spacing between the substrate surface and a donor sheet or a mask.
The potential problems or constraints also apply to disclosures by Grande et al. in commonly assigned U.S. Pat. No. 5,871,709 which describes a method for patterning high-resolution organic EL displays, as well as to teachings by Nagayama et al. in U.S. Pat. No. 5,742,129 which discloses the use of shadow masking in manufacturing an organic EL display panel.
The above described potential problems or constraints are overcome by disclosures of Tang et al. in commonly assigned U.S. Pat. No. 6,066,357 which teaches methods of making a full-color OLED display. The methods include ink-jet printing of fluorescent dopants selected to produce red, green, or blue light emission from designated subpixels of the display. The dopants are printed sequentially from ink-jet printing compositions which permit printing of dopant layers over an organic light-emitting layer containing a host material selected to provide host light emission in a blue spectral region. The dopants are diffused from the dopant layer into the light-emitting layer.
The ink-jet printing of dopants does not require masks, and surfaces of ink-jet print heads are not contacting a surface of the organic light-emitting layer. However, the ink-jet printing of dopants is performed under ambient conditions in which oxygen and moisture in the ambient air can result in partial oxidative decomposition of the uniformly deposited organic light-emitting layer containing the host material. Additionally, direct diffusion of a dopant, or subsequent diffusion of a dopant, into the light-emitting layer can cause partial swelling and attendant distortion of the domains of the light-emitting layer into which the dopant was diffused.
OLED imaging displays can be constructed in the form of so-called passive matrix devices or in the form of so-called active matrix devices.
In a passive matrix OLED display of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example, indium-tin-oxide (ITO) anodes are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Three or more organic layers are then formed successively by vapor deposition of respective organic materials from respective vapor sources, within a chamber held at reduced pressure, typically less than 10−3 Torr (1.33×10−1 Pa). A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Such conventional passive matrix OLED displays are operated by applying an electrical potential (also referred to as a drive voltage) between an individual row (cathode) and, sequentially, each column (anode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active matrix OLED display, an array of sets of thin-film transistors (TFTs) is provided on a light-transmissive substrate such as, for example, a glass substrate. One TFT, respectively, of each of the sets of TFTs is connected to a corresponding light-transmissive anode pad, which can be made, for example, of indium-tin-oxide (ITO). Three or more organic layers are then formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix OLED display. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix OLED display is described in commonly assigned U.S. Pat. No. 5,550,066.
In order to provide a multicolor or a full-color (red, green, and blue subpixels) passive matrix or active matrix OLED display, color pixelation of at least portions of an organic light-emitting layer is required.
Color pixelation of OLED displays can be achieved through various methods as detailed above. One of the most common current methods of color pixelation integrates the use of one or more of the described vapor sources and a precision shadow mask temporarily fixed in reference to a device substrate. Organic light-emitting material, such as that used to create an OLED emitting layer, is sublimed from a source (or from multiple sources) and deposited on the OLED substrate through the open areas of the aligned precision shadow mask. This physical vapor deposition (PVD) for OLED production is achieved in vacuum through the use of a heated vapor source containing vaporizable organic OLED material. The organic material in the vapor source is heated to attain sufficient vapor pressure to effect efficient sublimation of the organic material, creating a vaporous organic material plume that travels to and deposits on an OLED substrate. A variety of vapor sources based on different operating principles exist, including the so-called point sources (heated small sublimation cross-sectional area sources) and linear sources (sources of large sublimation cross-sectional area). Multiple mask-substrate alignments and vapor depositions are used to deposit a pattern of differing light-emitting layers on desired substrate pixel areas or subpixel areas creating, for example, a desired pattern of red, green, and blue pixels or subpixels on an OLED substrate. Note that in this method which is commonly used in OLED production not all of the vaporized material present in the vaporous material plume is deposited onto desired areas of the substrate. Instead much of the material plume is deposited onto various vacuum chamber walls, shielding, and precision shadow masks. This leads to poor material utilization factors and consequently high materials cost.
While precision shadow masking is a feasible method for OLED production, it also effects many complications and potential predicaments to display manufacturing. First, care must be taken in positioning and removing these masks onto and from a device substrate to avoid physical damage to OLED devices. Second, when vacuum depositing large area substrates it is difficult to keep shadow masks in intimate contact at all places along the length of the substrate, which can lead to unfocussed depositions or mask induced substrate physical damage. Third, when vacuum depositing three colored regions at different locations on the substrate, three sets of precision shadow masks may be needed and can cause unwanted delays in OLED production. Fourth, keeping mask to substrate precision alignment with the required accuracy along the length of large substrates is very difficult for several reasons, including mask and substrate thermal expansion mismatches, small pixel pitches, and mask fabrication limitations. Also, when vacuum depositing multiple substrates during a single vacuum pump down cycle, material residue can build up on shadow masks and can eventually cause defects to form in the pixels being deposited.